This article, Be Assertive by Dr David Horgan, had some good rationale on assertiveness.
The leading principle – ‘don’t be nice all the time, be fair (including to yourself)‘, warns of the anger, resentment and negativity that can build up from being continuously timid and lacking in assertiveness. This is one I know pretty well!
Other principles include:
People cannot read your mind – just because we do nice, thoughtful things for other people does not mean they will think of doing nice, thoughtful things for us! It is unrealistic to expect it. Most people expect that if we want something, we will ask.
They may not like me – because we want other people to like us it is sometimes hard to say ‘no’ to them. By continuously doing favours without asserting our own needs or rights we are letting the world know that it is okay to treat us as a doormat!
Assertiveness is a species protective mechanism – this follows from the principle above; being assertive may not always be pleasant but it does alter other people’s perception of us as a confident and strong person, which leads to being treated differently (hopefully better!).
You do not have to explain – in most situations where we decide to say no to a request, there is no law which says we have to justify our decision. It is simply exercising our own free will! In a tough situation it may be helpful to remember that you do not have to say yes or no immediately!
Do say what you think – if someone is more adamant in their opinion it doesn’t follow that your opinion is any less valid! (unless perhaps if the other person is an expert in the area). It is useful to say something like “I understand your point of view and I will think about it, but I would like you to think about my point of view also”.